Understanding how secretin gene expression is restricted to secretin cells in the gut is an attractive model for examining transcriptional control of intestinal differentiation. A proximal enhancer in the secretin gene has binding sites for the basic helix loop helix (bHLH) protein, BETA2, and two widely expressed proteins, Sp1 and Finb/RREB-1. BETA2 may function as a master regulator of secretin cell differentiation, coordinating transcriptional activation with cell cycle arrest. In addition, the transcriptional activity of BETA2 is enhanced through interactions with Finb/RRREB-1 and Sp1. Finb/RREB1 by itself does not increase transcription and may represent a new class of transcription modifying protein distinct from activators and cofactors, by functioning as a potentiator of BETA2. Since BETA2 is the only factor binding to the enhancer expressed in a small number of tissues, additional important regulatory domains are likely to exist further upstream in the secretin gene. The goal of the proposed research is to examine the transcriptional control mechanisms for secretin gene expression. The first aim will study how Finb/RREB-1 functions as a potentiator of BETA2, by examining 1) structural domains required for their interaction, 2) the role of ubiquitously bHLH partners of BETA2, and 3) the functional importance of two candidate Finb cofactors, CtBP and the PIAS proteins. Aim 2 will examine how Sp1 and BETA2 functionally synergize on the secretin promoter by characterizing how they associate and whether they form a higher order DNA-protein complex. In Aim 3, distal 5' regions in the secretin gene that direct expression in secretin cells will be identified by combining studies in transgenic mice with transfection studies in secretin-expressing cell lines. Factors binding to the elements will be biochemically characterized, cloned, and tested for their ability to activate or repress secretin gene expression. The opportunity to study differentiation in a dynamic, self-renewing population of endocrine cells from the gut endoderm, like secretin cells may have important implications for future therapies for disorders of hormone production such as diabetes mellitus. Information gained may facilitate manipulation of a proliferating progenitor pool to differentiate into cells capable of secreting hormones like, insulin and may potentially overcome limitations in the number of a cells, which do not proliferate, available for transplantation.